I haven't found myself this hooked on a puzzle game since Portal. And before that, possibly Lemmings. Dead simple concept, but countless variations and twists upon. Surprisingly good music, cartoony-but-loverly graphics, and some cheesy humour behind it all.

$20 on Steam (only 70MB download), probably going to hit stores soon if it hasn't done so already, and no DRM. Seriously, you owe it to yourselves to check this game out.